convert to string, then reverse the string.

char* itoa(int n) {
    char* ret = NULL;
    int numChars = 0;
    // Determine if integer is negative
    bool isNegative = false;
    if (n < 0) {
        n = -n;
        isNegative = true;
        numChars++;
    }
    // Count how much space we will need for the string
    int temp = n;
    do {
        numChars++;
        temp /= 10;
    } while ( temp );
    //Allocate space for the string (1 for negative sign, 1 for each digit, and 1 for null terminator)
    ret = new char[ numChars + 1 ];
    ret[numChars] = 0;
    //Add the negative sign if needed
    if (isNegative) ret[0] = '-';
    // Copy digits to string in reverse order
    int i = numChars - 1;
    do {
        ret[i--] = n%10 + '0';
        n /= 10;
    } while (n);
    return ret;
}

char* itoa(int x) {
    int i = x < 0 ? 3 : 2;
    int t = abs(x);
    while (t = t / 10) ++i;
    char* s = new char[i];
    s[--i] = '\0';
    t = abs(x);
    while (i) s[--i] = '0' + (t % 10), t = t / 10;
    if (x < 0) s[0] = '-';
    return s;
}

#include<stdio.h>
#include<stdlib.h>


void reverse(char *str,int len) {
	int i=0;
	char ch;
	for(i=0;i<=(len-1)/2;i++) {
		ch=str[i];
		str[i]=str[len-1-i];
		str[len-1-i]=ch;
	}
}

char* itoa(int number) {
	char *str=malloc(sizeof(char)*20);
	int negFlag=0,pos=0;
	if(number<0) {
		negFlag=1;
		number=-number;
	}
	while(number>0) {
		str[pos++]='0'+number%10;
		number=number/10;
	}
	if(negFlag) {
		str[pos++]='-';
	}
	str[pos]='\0';
	reverse(str,pos);
	return str;
}

int main() {
	printf("reverse %d : %s ",-543,itoa(-543));
	printf("reverse %d : %s ",54,itoa(54));
	printf("reverse %d : %s ",5,itoa(5));
}

you guys should also consider base, how about binary? decimal? hex?

int myitoa(char *num)
{
int i =0;
while( *num )
{
i = i<<3 + i<<1 + (*s - '0') ;
s++;
}
return i;
}

int myitoa(char *num)
{
int i =0;
while( *num )
{
i = i<<3 + i<<1 + (*s - '0') ;
s++;
}
return i;
}